1. Field of the Invention
This invention relates generally to steering systems for vehicles having steerable front and rear wheels. More particularly, this invention relates to a servo-controlled multi-mode steering system wherein the steered wheels turn in concentric arcs.
2. Description of the Prior Art
In order to exhibit favorable maneuverability in restricted zones, a wheeled land vehicle should have a multi-mode steering system which permits an operator of the vehicle to readily select the desired steering mode. Front wheel steering offers the best vehicle stability at higher speeds, but front wheel steering alone is unsuitable for extremely short radius turns and does not permit sidewise or crabbing movement of the vehicle in which all the wheels turn in the same direction. When a vehicle is turning through an arc having a short radius, all the steered wheels should turn through concentric arcs to reduce wear on the vehicle and on the road surface. Short radius turns are best accomplished by a steering system which turns all the wheels of a vehicle in concentric arcs.
Several known prior art devices have multi-mode steering systems which rely on heavy, complex mechanical steering linkages between a steering device and the steered wheels. U.S. Pat. No. 3,532,178 to Lindbom teaches a servo-controlled steering system which permits all the wheels of a vehicle to be turned in concentric arcs in the coordinated steering mode but which does not provide other steering modes. U.S. Pat. No. 3,856,102 to Queen teaches an electro-hydraulic multi-mode steering system which does not provide for the turning of the steered wheels in concentric arcs in the front wheel and coordinated steering modes. U.S. Pat. No. 3,572,458 to Tax teaches a servo-controlled multi-mode steering system wherein cams control the angles through which the steered wheels turn; however, the cams must be replaced in order to change the steering mode.
Many servo-controlled steering systems permit the operator of the vehicle to turn the steering wheel faster than the steered wheel is able to follow, and this lack of road feel makes precise steering impossible even at low speeds.
Thus a need exists for a precisely controllable multi-mode steering system which is not excessively heavy and complex and which may be readily switched from one steering mode to another.